Transparent conductive substrates that each include a transparent base, such as glass, and a conductive metal oxide film formed thereon are used for various devices, with a functional thin film being formed on the metal oxide film. Examples of such devices include: photoelectric conversion elements such as optical sensors, solar cells, etc.; image displays such as liquid crystal displays, organic electroluminescence (EL) displays, plasma displays, field emission displays (FEDs), etc.; and light emitting devices such as light emitting diodes, solid state lasers, etc. The transparent conductive substrates may be used also as Low-E (Low-Emissivity) glass, electromagnetic wave shielding glass, defogging glass, etc. for windows of buildings or vehicles, doors of refrigerators for stores, etc. The transparent conductive substrates are useful also as the document glasses of copying machines.
A photoelectric conversion element converts optical energy into electric energy or vice versa. For example, a silicon-thin-film solar cell has a configuration in which a silicon film with a photoelectric conversion function and an electrode film (a back electrode film) are formed sequentially on a metal oxide film of a transparent conductive substrate. The silicon-thin-film solar cell converts solar energy into electric energy. The losses caused by the reflection and absorption of light by a transparent conductive substrate as well as the electrical resistance of a metal oxide film decrease the efficiency of conversion of solar energy into electric energy.
For the respective uses that are typified by the photoelectric conversion element, the transparent conductive substrate is required to have both high optical transparency and high conductivity. It is easy to improve each of the optical transparency and the conductivity independently but it is not easy to improve both of them in a well-balanced manner.
It has been well-known to add a suitable dopant, for instance, fluorine, to a metal oxide film, for example, a tin oxide film, to lower the electrical resistance. The dopant improves the conductivity of the metal oxide film but deteriorates the optical transparency of the metal oxide film.
The conventional proposals about improvements in properties of the tin oxide film are described in JP62(1987)-211966A, JP62(1987)-213281A, JP1(1989)-96044A, and JP9(1997)-40442A, for example. The improvements achieved conventionally, however, were insufficient in terms of improving both the optical transparency and the conductivity in a well-balanced manner.